1. Field of the Invention
The present invention related to a grafted and cross-linked polyolefin material and a process for making it. More particularly, it relates to the use of this material as a separator in batteries, especially alkaline batteries.
2. Prior Art
Recent years have seen the rapid development of various alkaline battery systems. These systems include silver-zinc, mercury-zinc, nickel-cadmium, silver-cadmium, manganese-zinc, and mercury-cadmium. The silver-zinc cells are of particular interest as they exhibit a high energy to weight ratio. Partly for this reason, silver-zinc cells have found particular application in military, commerical and aero space uses. Alkaline cells typically require a separator between the anode and cathode to effect a physical barrier to prevent internal shorting. A great deal of effort has been expanded in the search for an appropriate separator for alkaline cells. The separator must be permeable so that ions can travel from one plate to another. As noted above, the separator must have sufficient physical strength to prevent internal shorting. Further, the separator should not so impede the progress of ions migrating through the electrolyte so as to markedly raise the impedance of the cell. Finally, the separator material must be capable of withstanding the chemically hostile environment produced by the combination of the electrolyte (typically a strong caustic such as potassium hydroxide or sodium hydroxide) and oxidation due to strong oxidizers such as divalent silver which is often used as a positive electrode.
One substance which has found wide use as a separator is modified polyethylene film. In order to effect the migration of ions within the cell, the polyethylene film is often graft polymerized using an ethylenically unsaturated monomer whereby carboxyl groups are grafted onto the polyethylene film and act as carriers for ionic charge through the separator material. In some cases, porous polyethylene sheet is treated with methacrylic acid and laminated between sheets of cellophane. The latter type of separator while effective is quite brittle and fractures easily making it difficult to handle and form. The former type of grafted separator conventionally is in the form of a thin film of polyethylene which is exposed to activated radiation and submerged in a solution of methacrylic acid. Because of the cross-linking of the polyethylene film, the extent of grafting is limited by the degree of swelling which the film is capable of undergoing. Therefore the bulk of the separator is substantially free of grafts. To date, most polyethylene separators (both with and without cellophane laminates) are undesirable brittle. The resistances of other type separators through oxidation is not satisfactory with the resultant shelf-life of the alkaline cell incorporating them unduly short.
It would be highly desirable to provide an improved separator for alkaline cells which has none of the foregoing deficiencies. It is therefore an object of the present invention to provide a material useful as a separator for alkaline cells which is physically tougher than those previously produced. A further object of the invention is to provide a material for an alkaline cell separator having a substantially improved shelf life. Yet another object is to provide a material and process for forming an improved alkaline cell separator. Another object is to provide a material and method of preparing a cell separator which exhibits superior resistance to the powerful oxidizing action of divalent silver oxide cathodes. A further object is to provide a material and method of preparing a separator which because of its oxidative stability allows the realization of the full capacity of the electrochemical couple.